1. Field of the Invention
The present invention relates to a fixed bit-rate encoding method and apparatus therefor, and a tracking method for performing a high speed search using the same. More particularly, it relates to a fixed bit-rate encoding method and apparatus therefor in which the high quality of an image is maintained during normal reproduction and the capacity of a high speed search is improved by fixing the code bit-amount in a cluster unit, and to a tracking method for the high speed search.
The instant application is based on Korean Patent Application Nos. 94-18079 and 95-6219 which are incorporated herein by reference for all purposes.
2. Discussion of Related Art
Recently, in an apparatus for transmitting/receiving image and audio signals, a technology has been used, whereby image and audio signals are transmitted, or stored in a memory, after encoding the image and audio signals as digital signals. Then, the signals are reproduced through a decoding operation. However, when the image signal is encoded as digital data, the amount of data is massive. Here, various encoding techniques are performed to reduce the total amount of data by removing redundancy in the digital image signal data. These techniques include transform coding, differential pulse code modulation, quantization, and variable-length encoding.
On the other hand, a magnetic recording density technology for high- density recording of the signal on a magnetic tape has been developed. However, due to the limitations of the magnetic recording density technology, it is difficult to record long segments of the digital image signal. Although, a digital videocassette recorder (DVCR) for compressing the digital image signal using the above-described data compressing method to increase recording time has been put to practical use.
The DVCR, however, generates many problems during high speed search operations, compared with an analog videocassette recorder. That is, in the case of a videocassette recorder adopting a helical scan method, since a reproducing head travels across a plurality of recording tracks for scanning during a high speed search, the signals recorded on the recording track are not completely reproduced. That is, only the data included in a portion of the tape held in contact between the recording track and head is reproduced. As a result, the quality of the image reproduced during the high speed search is deteriorated.
FIG. 1 shows the trace pattern of a head according to a conventional high speed searching method. In FIG. 1, the solid-lined arrows represent the head tracing path during a normal reproducing operation and the dotted-lined arrows represent the head tracing path during a triple-speed search operation. According to the high speed searching method shown in FIG. 1, since only a portion of each track scanned by the head is reproduced, only a partial image signal is reproduced.
FIG. 2 shows the trace of a head according to a conventional dynamic tracking method.
In FIG. 2, the head scans the first recording track along the first solid-lined arrow and passes two recording tracks along the dotted-lined arrows. Then, the head scans the fourth recording track. The triple-speed search is performed by repeating the above procedure.
Here, the dynamic tracking prevents deviation of the head from the recording track by changing the head position along the width of the track during the high speed search. Since the head is attached on the drum via a piezoelectric element or linear voice coil in the case of dynamic tracking, which is different from a general head which fixedly attached to a rotary drum, the head displacement is controlled by varying a voltage applied to the piezoelectric element or linear voice coil.
When N-times high speed searching is performed according to the dynamic tracking method, the head is controlled to scan one track (here, the first track) of the N continuous recording tracks. The capacity of the high speed searching method using the dynamic tracking is affected by the changeable displacement of the head which is determined by a mechanical structure. For example, supposing that a track pitch is W and the displacement of the track is W(N-1), then an N times high speed search is possible.
However, the head should be controlled from a position of zero to a displacement of W(N-1) in order to perform an N-times high speed search. Accordingly, the displacement value should be increased together with an increase in the searching speed for a DVCR system adopting the dynamic tracking method.
FIG. 3 shows head displacement according to the dynamic tracking method shown in FIG. 2.
In FIG. 3, a solid line A represents the trace of a head according to the dynamic tracking method and a solid line B represents the trace of a head according to the method shown in FIG. 1. As shown in FIG. 3, the head should move from a position of zero by a displacement of 2W (determined by W(N-1)), in order to perform triple-speed searching according to the dynamic tracking method. Thus, in the dynamic tracking method, the displacement value should be increased with an increase in searching speed. However, such an increase is limited by a mechanical structure, which causes a problem in that the searching speed of the dynamic tracking is restricted to a predetermined range. When recording a high definition television signal (HDTV) on a DVCR, various limitations are accompanied by requirements for a high quality image, inter-frame editing, multiple copying, quick search, etc. To satisfy the above requirements, it is important for a predetermined amount of the compressed image signals measured in bits, to be maintained in a frame unit.
However, since the size of a frame image varies according to the particular image characteristics, an effective compression encoding method is required in the DVCR in which recording should be performed using frame units of a predetermined size, called a bit-amount, or in other words, the number of bits. Also, a bit-amount fixing unit (a code-amount fixing unit) related to image quality is contradictory to the capacity of the high speed search. Further, it is difficult for the compression encoding method having a predetermined bit-amount to be performed in the case where variable-length encoding should be employed. That is, a small bit-amount fixing unit is good for high speed searching, but lowers the efficiency of a variable-length encoder, thereby deteriorating the image quality.
On the other hand, a general bit-amount control method used for a moving picture, such as a feedback control for buffer regulation to prevent the compressed signal amount from exceeding an allowable value, cannot be used without modification in the DVCR for HDTV signals recorded and reproduced by intra-frame encoding. Therefore, an effective bit-amount fixing method is required which guarantees the performance of the high speed search, as well as providing for a high quality image during normal reproduction.